On the basis of embryonic AP receptor localization with labeled ligand autoradiography, blood vessels are one group of peripheral tissue enriched in SP receptors initiated from gestation days 13-17. There are reports indicating that SP has hypotensive activity; therefore studies were conducted to examine the difference between the two strains of rate Wisto Kyoto (WKY) and stroke prone spontaneously hypertensive (SHRSP) in the distribution and chemical characteristic of SP receptors in their aorta. A significantly higher Bmax was observed in aorta of WKY in our preliminary studies. The result corresponds well with the reported hypotensive activity of SP. Further studies of the differences in chemical and physical characteristics of SP receptors between the two strains have been initiated.